This invention relates to a method of making bonded components, and more particularly to a method of making weldless magnesium/aluminum bonded components.
Diffusion bonding, also known as diffusion welding, is a solid state process in which joining two substrates is accomplished without a liquid interface (brazing) or the creation of a cast product by melting and resolidification (welding). Diffusion bonding produces solid state coalescence between two materials under several conditions. First, joining occurs at a temperature below the melting point of the materials to be joined. Second, coalescence of contacting surfaces is produced with loads below that which would cause macroscopic deformation of a part. Third, optionally a bonding aid may be used, such as an interface foil or coating, to either facilitate bonding or prevent the creation of brittle phases between dissimilar materials.
The sequence of metallurgical stages in diffusion bonding include: (a) initial contact which typically is limited to a few asperities at room temperature; (b) deformation of surface asperities by plastic flow and creep; (c) grain boundary diffusion of atoms to the voids and grain boundary migration; and (d) volume diffusion of atoms to the voids.
In diffusion bonding, there is usually little permanent deformation of the bulk of the parts being joined, although local deformation does occur at their interface on a microscopic scale. In some circumstances, interfacial contamination, such as oxygen, may interfere with the bonding mechanisms. In these circumstances, the process is usually conducted in an inert atmosphere such as in a vacuum or in the presence of an inert gas.
Some superplastic materials are ideally suited for processing by diffusion bonding because they deform easily at the superplastic temperature, which is a temperature consistent with that required for diffusion bonding. Typically, these alloys tend to have a high solubility for oxygen and nitrogen, so the contaminants often are removed from the surface by diffusion into the base metal. For example, titanium alloys fall into this class and are easily diffusion bonded. However, aluminum alloys form a very thin but tenacious oxide film and are therefore quite difficult to diffusion bond.
The process conditions under which superplastic forming and diffusion bonding are carried out are quite similar. Both require an elevated temperature and take advantage of the benefits from fine grain size. Consequently, the combined process of superplastic forming and diffusion bonding has heretofore been developed.
As indicated earlier, aluminum and its alloys are difficult to join by diffusion bonding because of tightly adhering oxide film that naturally develops on the aluminum. However, it is known that once the oxide has been removed, the diffusion bonding process can take place at temperatures ranging from 454xc2x0 C. to 538xc2x0 C. Other metals that have been known to be joined by the diffusion bonding process include beryllium and its alloys, copper and its alloys, heat-resistant cobalt and nickel alloys, various steels, columbium and its alloys, tantalum and its alloys, titanium and its alloys, and zirconium and its alloys. The diffusion bonding of aluminum and magnesium or aluminum alloys and magnesium alloys has not heretofore been known.
One method of increasing fuel economy for vehicles is to manufacture and assemble the vehicles using lightweight materials. However, the use of lightweight materials for automobile structures may be limited by material cost which at this date makes substituting aluminum or magnesium for steel cost prohibitive. Thus, new methods of utilizing lightweight materials that reduce the manufacturing and assembly costs are very desirable. The present invention satisfies this need in the industry.
One embodiment of the present invention is a method including the steps of contacting at least a portion of a first substrate with at least a portion of a second substrate. The first substrate includes at least 50 weight percent aluminum and the second substrate includes at least 50 weight percent magnesium. The first and second substrates are heated at an elevated temperature above 440xc2x0 C. Pressure is applied to the first and second substrates at least at one point of contact to bond the first and second substrates together.
Another embodiment of the present invention is a method including the steps of contacting at least a portion of a first substrate with a portion of a second substrate. The first substrate includes at least 90 weight percent aluminum, and the second substrate includes at least 85 weight percent magnesium. The first and second substrates are heated together at an elevated temperature ranging from about 440xc2x0 C. to 500xc2x0 C. Pressure is applied to the first and second substrates at least at one contact point to bond the first and second substrates together.
Another embodiment of the present invention includes a step of contacting at least a portion of a first substrate with at least a portion of a second substrate. The first substrate comprises at least 90 weight percent aluminum and a first set of additives comprising at least one metal. The second substrate comprises at least 85 weight percent magnesium and a second set of additives comprising at least one metal. The first and second substrates are heated to an elevated temperature ranging from about 440xc2x0 C. to 500xc2x0 C. Pressure is applied to the first and second substrates at least at one point of contact to bond the first and second substrates together. The first set of additives may include magnesium. Preferably, the first set of additives includes a plurality of metals including magnesium, and wherein magnesium is present in the highest concentration of the plurality of metals in the first set of additives. Most preferably, the first substrate comprises about 4.5 weight percent magnesium.
In another embodiment of the invention, the second set of additives includes aluminum. More preferably, the second set of additives includes a plurality of metals including aluminum, and wherein aluminum is present in the greatest concentration of the plurality of metals in the second set of additives. Most preferably, the second substrate includes about 3 weight percent aluminum. The second substrate may also include aluminum and zinc.
In a preferred embodiment of the invention, the first substrate comprises at least 95 weight percent aluminum and at least 90 weight percent magnesium.
Another embodiment of the invention is a method including the step of contacting at least a portion of a first substrate with at least a portion of a second substrate. The first substrate comprises at least 90 weight percent aluminum and a first set of additives comprising at least one metal. The second substrate comprises at least 85 weight percent magnesium and a second set of additives comprising at least one metal. The first and second substrates are heated to an elevated temperature ranging from about 440xc2x0 C. to 500xc2x0 C. Pressure is applied to the first and second substrates at least at one point of contact to bond the first and second substrates together. Pressure is applied to other portions of the first and second substrates to superplastically form the first and second substrates into a desired shape.
These and other objects, features and advantages of the present invention will become apparent from the following brief description of the drawings, detailed description of the preferred embodiment and appended claims and drawings.